I want to build a circuit (may I say, larger circuit) that will have some interaction with the PC. What comes to my mind is some kind of 'meter, but I need some interesting project to keep me busy for some time.

Build yourself a software-based oscilloscope. It's pretty easy, and gives you a very handy tool.

You'll need to make an interface box that connects your sound card inputs to test leads, in a way that protects the sound card. (Be extremely cautious when connecting one power source to another. If they're not isolated, you can pass current over just a single ground wire.) I've posted a picture of my interface box here somewhere - I'll see if I can find it if you're interested.

Build yourself a software-based oscilloscope. It's pretty easy, and gives you a very handy tool.

You'll need to make an interface box that connects your sound card inputs to test leads, in a way that protects the sound card. (Be extremely cautious when connecting one power source to another. If they're not isolated, you can pass current over just a single ground wire.) I've posted a picture of my interface box here somewhere - I'll see if I can find it if you're interested.

Click to expand...

Why not? One thing though - I read somewhere that it is unable to measure digital signals.. However.., please post the pic if you can find it.

If instead of "digital" you mean "fast", then yes, it cannot do that. I'm sure sound cards differ but my old Mac starts to show not-so-smooth curves when fed a sine wave over 5kHz or so. It also fails on the low end when the signal is nearly DC. At, say, 5Hz and below, a square wave will visibly tail off as the charge on the coupling capacitor (on the sound card's input) fades. Still very useful but you have to not get excited about the apparent odd shape.

I think you will find it cant measure 'DC', not digital, signals because a sound card will be AC coupled, IE its inputs have a series capacitor.
(Sorry didnt read the whole post above - Not DC, you said that)

I would suggest that you get hold of an external or PCI sound card so that you can take the capacitors out and build a new input stage to the ADC circuits.
If you google sound card oscilliscope you will find quite a bit of info.

Bear in mind you will need to write software that directly reads the ADC's and the ability to do this may vary from card to card.

The easiest interface to manipulate on a modern PC is the parallel port, there are free controls / library's for most languages.

There is enough accessible IO on a parallel port in EPC mode to control a small robot arm.

You could also look at serial IO, RS232 is speciffied as +/-12 volts but many PC's implement the same protocol using 0-5 volts. 232 is also a point to point interface but again with a little ingenuity, working in 3 wire asynch mode you can make it work as a multy-drop network much like RS485.

Have a look at Modbus IO circuits, they will give you an idea of what can be done relatively easily.

There are a LOT of existing solutions and unless you specifically WANT a programming project, you could avoid it entirely if your goal is an oscilloscope. If you want to build a robot girlfriend, well, that's a bit more complicated.

Hi again.. while I'll keep the o'scope idea in mind, I think I found something interesting to make. I want to have a computer operated timer, (i.e., the countdown interval is set by the PC,) and when the countdown comes to 0, to have a output signal (0 to 1 change maybe). Something else shall be surely added later, but this is the basic idea.

Depending on your future plans, you might like to look into a LabJack data acquisition device. I have a U3-HV and love it. It gives you a USB interface to ...the world. It's got all sorts of inputs and outputs and is easy to control with almost any language. It can even be controlled over the internet (hosted by a computer, of course).

It would be easy to do what you want with such a thing.

I say "future plans" because it's about $120, which is likely overkill for this particular project. But you'll get value out of it for years.

Well, that would drain the whole fun of the project (kinda changed my mind about using finished software). I actually thought using parallel port to communicate with the counters, which IMO is not very hard to perform, and would be quite interesting. I have some doubts about this, though, but I might open a new topic about it, unless, you are interested to help me.